1. Technical Field of the Invention
This invention relates generally to wireless communication systems and more particularly to local oscillation generators of wireless communication devices operating in such wireless communication systems.
2. Description of Related Art
Communication systems are known to support wireless and wire lined communications between wireless and/or wire lined communication devices. Such communication systems range from national and/or international cellular telephone systems to the Internet to point-to-point in-home wireless networks. Each type of communication system is constructed, and hence operates, in accordance with one or more communication standards. For instance, wireless communication systems may operate in accordance with one or more standards including, but not limited to, IEEE 802.11, Bluetooth, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), local multi-point distribution systems (LMDS), multi-channel-multi-point distribution systems (MMDS), and/or variations thereof.
Depending on the type of wireless communication system, a wireless communication device, such as a cellular telephone, two-way radio, personal digital assistant (PDA), personal computer (PC), laptop computer, home entertainment equipment, et cetera communicates directly or indirectly with other wireless communication devices. For direct communications (also known as point-to-point communications), the participating wireless communication devices tune their receivers and transmitters to the same channel or channels (e.g., one of the plurality of radio frequency (RF) carriers of the wireless communication system) and communicate over that channel(s). For indirect wireless communications, each wireless communication device communicates directly with an associated base station (e.g., for cellular services) and/or an associated access point (e.g., for an in-home or in-building wireless network) via an assigned channel. To complete a communication connection between the wireless communication devices, the associated base stations and/or associated access points communicate with each other directly, via a system controller, via the public switch telephone network, via the Internet, and/or via some other wide area network.
For each wireless communication device to participate in wireless communications, it includes a built-in radio transceiver (i.e., receiver and transmitter) or is coupled to an associated radio transceiver (e.g., a station for in-home and/or in-building wireless communication networks, RF modem, etc.). As is known, the transmitter includes a data modulation stage, one or more intermediate frequency stages, and a power amplifier. The data modulation stage converts raw data into baseband signals in accordance with a particular wireless communication standard. The one or more intermediate frequency stages mix the baseband signals with one or more local oscillations to produce RF signals. The power amplifier amplifies the RF signals prior to transmission via an antenna.
As is also known, the receiver is coupled to the antenna and includes a low noise amplifier, one or more intermediate frequency stages, a filtering stage, and a data recovery stage. The low noise amplifier receives inbound RF signals via the antenna and amplifies then. The one or more intermediate frequency stages mix the amplified RF signals with one or more local oscillations to convert the amplified RF signal into baseband signals or intermediate frequency (IF) signals. The filtering stage filters the baseband signals or the IF signals to attenuate unwanted out of band signals to produce filtered signals. The data recovery stage recovers raw data from the filtered signals in accordance with the particular wireless communication standard.
For direct conversion receivers and transmitters (i.e., those that convert RF signals to baseband signals, and vice versa, directly), the local oscillation has a frequency that substantially matches the frequency of the RF signals and is generated by a single local oscillation generator (LOGEN). Typically, the LOGEN will include a phase locked loop (PLL) that converts a reference clock to a higher frequency clock signal, which may be directly used as the local oscillation or manipulated to produce the local oscillation. For example, the PLL may produce a clock signal that is approximately ⅔rds the rate of the RF signals. The PLL clock signal may then be divided by 2 to produce a second clock signal that is approximately ⅓rd the rate of the RF signals. The ⅔rds clock signal and the ⅓rd clock signal are combined to produce a local oscillation that has a rate corresponding to the RF signals.
In many wireless applications, the RF signals may be conveyed in different channels within different frequency bands. For example, in an IEEE 802.11a application, the frequency bands include 5.15 to 5.35 GHz and 5.725 to 5.825 GHz. Each of these frequency bands supports multiple 20 MHz bandwidth channels. As such, to select a particular channel within a particular frequency band, the local oscillation is adjusted accordingly. For a PLL based LOGEN, the PLL is adjusted to produce the adjusted local oscillation.
As is known, a PLL has a limited range over which it can be accurately adjusted. Thus, for accommodating the frequency bands of a particularly wireless application (e.g., 802.11a) the PLL performs well. However, when a transceiver is a multi-mode transceiver (e.g., one that is compliant with multiple wireless standards) and the frequency bands are substantially different from standard to standard (e.g., 2.4 GHz for 802.11b and 5.25 GHz for 802.11a), the PLL does not have sufficient range to accurately accommodate both standards. As such, a multi-mode transceiver includes two, or more, local oscillation generators: one for a first standard (e.g., 802.11a), another for a second standard (e.g., 802.11b), etc.
When the transceiver is implemented primarily on an integrated circuit, the extra local oscillation generates consumes die area and power. For battery powered devices and/or handheld mobile devices, the never-ending goal is power reduction and size reduction. Thus, the multiple LOGENs are an impediment to achieving smaller integrated circuits and reduced power consumption.
Therefore, a need exists for a method and apparatus for generating multiple local oscillations for a single local oscillation generator.